Seismic Force Resisting System 2

[ThomasH]

Hi all

I have a couple of questions regarding the Seismic Force Resisting System (SFRS) in a building and how it is classified. We have a structure in Seismic Design Category D.

Say that this structure consists of two columns and a truss betweem them. The connection between the column and the truss can transfer moments, top and bottom chort are connected to the column. If I interpred AISC 341 "Sieimic Provisions for Structural Steel Buildings" correct it means that the truss have a maximum allowed span of 20 m and a max depth of 1.8 m. It's a "Special Truss Moment Frame". But if I instead change the bottom chord so that it does not met the columnn making the truss simply supported. We stabilize the building with bracings (SCBF), does that mean that the 20m/1.8m limits for the trusses does no longer apply?

A similair question: are all trusses with span larger than 20 m prohibited or just trusses in moment frames?

Another question is regarding the SFRS. Is there any clear explanation regarding what parts of a structure is included in the SFRS and what parts are not? Is all parts that shall "help" during the eartquake to be considerad av part of the SFRS?

What I'm after is primarily how the code applies for structures with large trusses.

Best Regards

Thomas

 

[KootK]

But if I instead change the bottom chord so that it does not met the columnn making the truss simply supported. We stabilize the building with bracings (SCBF), does that mean that the 20m/1.8m limits for the trusses does no longer apply?

Yes.

A similair question: are all trusses with span larger than 20 m prohibited or just trusses in moment frames?

Just trusses in moment frames.

Another question is regarding the SFRS. Is there any clear explanation regarding what parts of a structure is included in the SFRS and what parts are not? Is all parts that shall "help" during the eartquake to be considerad av part of the SFRS?

This comes down to judgement. If an element not intended to be a lateral resisting element is stiff enough that it will attract significant lateral load and materially affect response, it's best to either isolate it or recruit it formally to help out with lateral. Sometimes exceptions can be made for elements that would fail in a ductile manner if accidentally overloaded laterally.

What I'm after is primarily how the code applies for structures with large trusses.

Simply supported trusses are not considered part of the vertical shear force resisting system to my knowledge. There will be seismic loads on the trusses from tributary seismic weights and there are requirements for minimum connection forces at the supports but I think that's about it. Your truss may serve as the chord / drag strut in your diaphragm designs.

The limits for trusses used as moment frames are simply there to keep things within the scale of the testing that was done on this systems (Ann Arbor I think).

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ThomasH]

KootK

I really appreciate your answers. They certaily simplyfy things for us.

So as long as we keep the trusses simply supported or continuous but pinned in the columns there are no special requirements like 20 m span limitations or depth limitations? The seismic force is "just another force".

We are working with a structure that has requirements for large spanns and the 20 m limit is a problem for the process in the building.

The next issue is probably that since the structure is irregular in seme respects we are probably not allowed to use the simplified methods for analysis (like lateral force). But that is a solvable problem. I our design violates the code in terms of not being allowed, that would be a clear dealbreaker.

Thank you

Thomas

 

[KootK]

You're most welcome Tom.

So as long as we keep the trusses simply supported or continuous but pinned in the columns there are no special requirements like 20 m span limitations or depth limitations? The seismic force is "just another force".

Yesir. What kind of world would it be if your friendly neighborhood structural engineer couldn't supply a great big monstrous truss when required? Giant truss up a storm my good man. Do us proud.



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.